The present invention relates generally to providing electromagnetic interference (EMI) isolation of sensitive circuit areas on a circuit board and more particularly to a deflection-type gasket for providing EMI isolation by substantially attenuating EMI.
EMI is often encountered in the design of electronic circuits. Semiconductor devices such as metal oxide semiconductor field effect transistors (MOSFETs) and bipolar junction transistors (BJTs), which are widely used in electronic circuits, usually operate at frequencies ranging from several kilohertz to gegahertz. Unfortunately, operation at such frequencies produces EMI, which may cause malfunction in electronic equipment.
For example, an electronic device may contain several circuits some of which may generate EMI and others which do not generate EMI. Circuits which do not generate EMI may be adversely affected when EMI propagates to those areas. Circuits which generate EMI are herein referred to as EMI generating circuits and circuits which are affected by EMI propagation are herein referred to as EMI sensitive circuits.
EMI propagation between the EMI generating circuits and the EMI sensitive circuits is known as cross talk. Because it is necessary to prevent cross talk, both EMI generating circuits and EMI sensitive circuits are commonly shielded, and thus isolated from each other.
In one existing shielding arrangement, metal shield cans or lids are mounted on the printed circuit (PC) board to isolate the EMI generating circuits from the EMI sensitive circuits.
In another shielding arrangement, circuits which are susceptible to cross talk are fabricated on different circuit boards. The circuit boards are then mounted on a circuit board holder formed into a plurality of cells, each cell retaining a single circuit board. The circuit boards are thus isolated from each other in different cells of the circuit board holder.
These structures, while providing isolation between circuits susceptible to cross-talk, are complex, requiring additional material and manufacturing processes and are, therefore, costly to manufacture. Furthermore, they are inefficient in utilizing scarce space available on the circuit board and require the product into which they are incorporated to be larger to accommodate these larger, more complex structures. Efficient use of space can be a vital concern in some applications such as wireless communications and cellular telephony where smaller, more compact units or equipment are desired.
One prior art arrangement for isolating EMI generating circuits and EMI sensitive circuits is disclosed and claimed in U.S. Pat. No. 5,252,782 which describes a single board EMI isolation system comprising a "clam shell" to enclose either the EMI generating circuits or the EMI sensitive circuits or both on the circuit board. A simple compressible conductive gasket is positioned around the perimeter of the circuit to be isolated between the rim of the clam shell and the surface of the circuit board to enhance the EMI isolation.
While the above structure provides efficient utilization of space on the circuit board, it may fail to provide a satisfactory level of EMI attenuation because the circuits to be isolated must be substantially completely sealed off to prevent cross-talk. This is achieved by tightly securing the clam shell to the circuit board which could result in overly compressing the gasket between the rim of the clam shell and the circuit board. Typically, a plurality of fasteners are used to tightly seal such a clam shell and gasket configuration to a circuit board to provide effective EMI attenuation. If care is not taken during assembly, the fasteners may be over tightened or may require excessive tightening to provide an effective seal, causing the gasket to be overly compressed, and even possible deformation of the circuit board. In addition, excessive tightening of the gasket may result in a phenomenon known as "compression set" wherein the gasket permanently assumes a compressed state and loses its resilient property. These adverse conditions may result in gaps in the EMI shielding formed by the clam shell and gasket combination and resultant leakage of EMI between the EMI generating and EMI sensitive circuits.
Known prior art EMI isolation systems of the type just described utilize a gasket designed to be compressed to seal off the circuit areas susceptible to cross-talk and to achieve satisfactory attenuation of EMI propagation. These systems, however, only provide a single location or line of sealing contact where the gasket is compressed between the clam shell and the circuit board which can result in leakage or ineffective attenuation of EMI propagation from the EMI generating circuits to the EMI sensitive circuits, if not assembled correctly as just described.
In view of the foregoing disadvantages, there is a need for a new gasket structure which achieves a satisfactory level of EMI attenuation with a simple structure that is easy to assemble and less expensive to manufacture, and which does not cause compression of the gasket or deformation of the circuit board.